This paper provides data and evidence that petroleum contamination in a fractured karst aquifer at a site in central Missouri is naturally degrading anaerobically. In typical petroleum contaminated sites as groundwater moves into the source area dissolved oxygen is rapidly consumed leading to anaerobic conditions in and down gradient of the source. The anaerobic area tends to segregate into terminal electron acceptor process (TEAP) zones such as iron reducing, sulfate reducing and carbon dioxide reducing (methanogenic) zones. Fractured or karst aquifers are frequently considered too open to revert to anaerobic degradation. However, there are three types of porosity-permeability systems present in each fractured bedrock aquifer: 1) matrix porosity, 2) "normal" fracture porosity such as along joints or bedding planes, and 3) large conduits such as karst, faults or large open fractures. Types one and two are conducive to anaerobic conditions. Typically, both matrix and "normal or typical" fracture systems will tend to dominate storage and flow locally during non-storm conditions which is the majority of the time. Simple geochemical analyses of groundwater can determine if an aquifer is aerobic or anaerobic. A profile of dissolved oxygen, sulfate, and iron across a plume can be definitive in determining if anaerobic TEAP zones are present. Additionally, accurate redox readings in millivolts yield clear indications of given TEAP zones. Both groundwater redox potential and geochemical analyses show that in this fractured karst aquifer in central Missouri the hydrocarbon plume is degrading anaerobically. Both dissolved oxygen and sulfate are depleted in the plume area relative to background concentrations, and the oxidation-reduction potential (ORP) changes from a background of nearly +200 millivolts to -127 millivolts in the plume.

E. Calvin Alexander Jr., University of Minnesota
E. Calvin Alexander, Jr. is a Morse-Alumni Professor in the Geology and Geophysics Department of the University of Minnesota in Minneapolis and specializes in karst hydrogeology.